1. Field of the Invention
The present invention relates to cameras and image capturing methods, and particularly relates to an anti-flicker function to solve the flicker problem due to oscillated background illumination.
2. Description of the Related Art
To capture images by a camera, the background illumination may affect the captured images. Considering an environment with indoor lighting, dark and bright lines may be interlaced over an entire image captured by a camera because the power source of the background illumination may oscillate frequently (due to an AC power source). To solve the flicker problem of the captured image, anti-flicker technologies are required.
In conventional anti-flicker techniques, the exposure time for each line of an image is an integer multiple of the flicker period of the background illumination. FIG. 1A depicts a rolling shutter operation for a sensor array (e.g. CCD or CMOS array) and shows the oscillation of the background illumination. In the rolling shutter mode, the rows of the sensor array are exposed in turn rather than being exposed altogether for a snapshot of a single point of time. As shown, the exposure time for each row is 1/fbi second—the same as the flicker period of the background illumination. For every row, the integrated value of background illumination is constant. For example, the integrated value 102 of background illumination collected during the exposure of the kth row equals the integrated value 104 of background illumination collected during the exposure of the lth row. Thus, the flicker problem is solved.
However, for high-brightness background illumination, the exposure time for each row has to be cut even shorter than the flicker period (1/fbi sec) of the background illumination. FIG. 1B shows the integrated value of background illumination not being constant for every row when the exposure time for each row is reduced to half the flicker period of the background illumination—1/(2fbi) sec. For example, the integrated value 106 of background illumination collected during the exposure of the kth row is much more than the integrated value 108 of background illumination collected during the exposure of the lth row. The flicker from the background illumination is still visible in the captured image.
Thus, an anti-flicker technique which is free from the restraint of the exposure time is called for, especially for high-brightness background illumination.